Vortex Breakdown Induced by an Adverse Pressure Gradient

Abstract

Understanding and predicting vortex behavior is crucial to maximize performance using aerodynamic design. Vortex breakdown can be induced to benefit the flow field or has to be prevented to avoid losing performance. It is therefore important to study the dynamics of vortex breakdown.

The main objective of this thesis was to assess the feasibility of measuring a free, streamwise vortex and its breakdown from data acquired with robotic volumetric Particle Image Velocimetry (PIV), whilst analyzing the relation between the local flow properties and the location of the breakdown. Experiments were conducted at the W-Tunnel at the TU Delft High-Speed Lab, where vortex breakdown was induced by the adverse pressure gradient field in front of a cylinder. The robotic Coaxial Volumetric Velocimeter (CVV) was used to assess the feasibility of using this system for vortex breakdown identification, in combination with data processing using a Lagrangian Particle Tracking (LPT) technique (Shake-The-Box), to detect and analyze vortices and their breakdown. The results of this experimental campaign were analyzed to identify vortices and their breakdown from the CVV data by characterizing the vortex’s internal structure. The analysis included establishing the relations of the adverse pressure gradient and the Reynolds number with the breakdown location.

The results showed that vortex breakdown is induced by the adverse pressure gradient. However, the breakdown location is not solely dependent on it. Furthermore, the effect of the Reynolds number at the breakdown location is barely measurable with the robotic volumetric PIV system.

This exploratory and experimental study showed that the CVV in combination with LPT can be used to recognize the structure of the vortex in both its pre- and post-breakdown regimes. However, for the analysis of vortex breakdown, it was found that the data was affected by the low spatial resolution. Thus, robotic volumetric PIV cannot provide conclusive results in terms of the relation between the location breakdown and local flow properties. Several recommendations regarding the experimental set-up are given for future research.